As a heat sensitive material for controlling temperature which determines whether the temperature reaches the predetermined temperature or for determining hysteresis of the temperature visually, a irreversible heat sensitive label is used heretofore. The heat sensitive label is formed by printing a heat-melt substance of a heat sensitive agent on a colored paper material. According to the heat sensitive label, the heat-melt substance melted by heat at predetermined temperature or more makes transparent or is absorbed into a paper material. Thereby, shielded color of the paper material is appeared and changes its hue to detect whether temperature reaches the predetermined temperature. Thus heat sensitive material can determine a result of the over-heated temperature than predetermined temperature under daylight condition simply and visually. However, it is unsuitable to control the temperature at a specific place such as dark, altitude, sealed or closed places, because it is hard to determine the hue visually.
An temperature control material which can control whether the temperature reaches the predetermined temperature or can determine hysteresis of the temperature without visual observation, is desired. For example, temperature control materials, which can detect it by electric procedures, have been developed. As thus temperature control materials, Patent Document 1 discloses a temperature control material wherein a heat-melt substance and a conductive paste including conductive powder are contacted and/or contiguous each other. According to this temperature control material, the heat-melt substance is melted by heat at temperature being detected, and then a conductive state thereof changes to be in an insulated or lower-conductive state. By using thus change, it can electrically detect whether the temperature reaches the predetermined temperature in order to control the temperature.
This temperature control material changes the conductive state to the insulated or lower-conductive state, by permeating the melted heat-melt substance by heat into the conductive paste and swelling the conductive paste. However, the heat-melt substance and the conductive paste in this temperature control material are contacted, and thereby the heat-melt substance gradually migrates and permeates into the conductive paste and makes the conductive paste swell. The conductivity of this temperature control material deteriorates with time as exceptionable phenomenon, even if it is not over the predetermined temperature. Since this temperature control material can be not used for long time stably, it has some problems of a short effective term or poor practicality. And metal powder used for the conductive paste is easy to cause decomposition by oxidation or sulfurization, and the conductivity is variable easily. Therefore this temperature control material is difficult to use in a hostile condition such as hot or humid conditions. Furthermore, in some case where the melted heat-melt substance by heat at the predetermined temperature or more indicates low viscosity, the heat-melt substance may flow out from the conductive paste and may not permeate or disperse into the conductive paste and the conductive paste may not swell sufficiently. Consequently, even if the temperature reaches the predetermined temperature, the conductivity thereof does not decrease and the temperature control material cannot detect it.
Therefore, a temperature control, material, which improves the problems and has more practicality and can determine whether the temperature reaches the predetermined temperature or can determine hysteresis of the temperature electrically to control it, is desired.